Field
The present invention relates to method for monitoring sleep phenomena. The present invention further relates to a system for monitoring sleep phenomena.
Description of the Related Art
Sleep may be characterized with two major types of sleep: Non-REM (Rapid Eye Movement) sleep and REM sleep, which is also known as dream sleep. Non-REM sleep consists of three stages of sleep having different depth starting from the lightest stage of transition to sleep to a second stage of light sleep, and deepening into third stage of deep sleep, where the sleeper is very difficult to awaken. Sleep occurs in series of recurring sleep stages, where the periods of deep restorative sleep and more alert stages alternate. These periods may be called as sleep cycles.
Sleep apnea (sleep apnoea) is an example of a sleep disorder, characterized by pauses in breathing during sleep. Such pauses may last for seconds or even minutes. When breathing is paused, carbon dioxide builds up in the human body, especially in the blood circulatory system. The brain is signaled to wake the person up by receptors in the blood stream detecting the high carbon dioxide levels, in order to make him/her breathing. Then the person falls asleep again. This kind of incidents may occur several times over the night and reduce significantly the quality of sleep, which may further cause various risks and/or be an important contributor in development of various health problems.
Hypopnea (hypopnoea) refers to another type of a sleep disorder, involving episodes of overly shallow breathing or an abnormally low respiration rate. It's typically defined by a decreased amount of air movement into the lungs, and causes drop of oxygen levels in the blood. Like sleep apnea, sleep is disturbed such that even if persons afflicted with hypopnea may get a full night sleep, they don't feel properly rested.
Ballistocardiography (BCG) is a measure of ballistic forces on the heart. It can be characterized as a mechanical response of the electrocardiographic signal (ECG). As heart pumps blood, two mechanical effects may be measured: motion of the heart causes a recoil effect on the chest, and motion of the blood causes a recoil effect in whole body. A ballistocardiographic (BCG) signal, which may also be called as a ballistocardioglogic signal, has a characteristic form, which is based on the blood flowing up and down in the body. This signal, for example delay and details of the shape of the BCG signal can reveal cardiac dysfunction. So called J-peaks of a BCG signal may be used to measure heart rate (HR) and heart rate variability (HRV) in a similar way as the R-peaks are used in the Electrocardiogram (ECG).
Ballistocardiographic data indicates the extent of mechanical movements of a body that take place in response to the myocardial activity of the heart. Such ballistocardiographic data may then be used to process data that is indicative of heart motion of the subject. Ballistocardiography based on accelerometer(s) or angular rate sensor(s) provides a non-invasive, unobstrusive and relatively lightweight method for measuring both the relative stroke volume of the heart and the beat-to-beat times.
Heart rate variability (HRV) refers to a variation in the beat-to-beat interval of the heart. Although the measured physiological phenomena is the same for HRV and beat-to-beat interval, typical parameters describing these are different. While beat-to-beat time is expressed typically in time scale, heart rate (HR) is typically expressed on a frequency scale, for example in beats per minute. Heart rate variability (HRV) may be expressed through indicating the relative rate change among a number of consecutive beats. Heart rate variability (HRV) may be calculated from detection of beat-to-beat intervals with a suitable data processing function. Variation in the beat-to-beat interval is a physiological phenomenon; the sinoatrial node of the heart receives several different inputs, and the instantaneous heart rate and its variation are results of these inputs. Recent studies have increasingly linked high heart rate variability (HRV) to good health and a high level of fitness, whilst decreased heart rate variability (HRV) is associated to stress and tiredness.
Analysis of heart rate variability (HRV) in the frequency domain is a widely used tool in the investigation of autonomic cardiovascular control. Usually the variability is differentiated in the spectral profile into the high frequency (HF) band (0.10 to 0.40 Hz), the low frequency (LF) band (0.04 to 0.10 Hz), and the very low frequency (VLF) band (<0.04 Hz). For example, breathing cycle causes a natural, clearly detectable variation of heart rate, where the R-R interval on ECG, and the J-J interval in BCG, is shortened during inspiration and prolonged during expiration. This variation is called as Respiratory Sinus Arrhythmia (RSA), which is detected in the high frequency (HF) band. The low frequency (LF) band (0.04 to 0.10 Hz) represents oscillations related to regulation of blood pressure and vasomotor tone including the so-called 0.1 Hz fluctuation. Heart rate variability in the low frequency (LF) band may be referred to as low frequency heart rate variability (LFHRV).
Stroke volume variability (SVV) refers to changes in arterial blood pressure induced by mechanical ventilation. Stroke volume variability (SVV) is a naturally occurring phenomenon in which the arterial pulse pressure falls during inspiration and rises during expiration due to changes in intra-thoracic pressure secondary to negative pressure ventilation (spontaneously breathing). Stroke volume variability (SVV) may be defined as the percentage change between the maximal and minimal stroke volumes (SV) divided by the average of the minimum and maximum over a floating period.
Respiration rate (RR) refers to rate of respiration of a subject. Although it's not a cardiologic measure, respiration rate (RR) may be detected using the same sensor(s) that are used for detecting ballistocardiologic signals, since respiration causes movement of the body of the subject detectable with accelerometer(s) and/or angular rate sensor(s). Respiration rate (RR) may also be obtained indirectly from a stroke volume variation (SVV) signal, since stroke volume (SV) signal amplitude is modulated by respiration.
Patent publication WO2010145009 discloses an apparatus for determining information indicative of physiological condition of a subject. The apparatus comprises an accelerometer sensor device that obtains ballistocardiographic data indicative of heart motion of the subject, measured along a plurality of spatial axes.
Article “MEMS accelerometers target healthcare applications” by Ulf Meriheinä in Electronic Engineering Times Europe, March 2013, pp. 40-41, discloses utilization of relative beat volume variation and beat-to-beat time variation for monitoring recovery state or stress level of a subject.
Various sleep disorders are typically diagnosed in sleep laboratories, using a combination of a variety of measurements, including electroencephalogram (EEG), respiration flow or respiration muscle status, muscle activity and electrocardiogram (ECG). Such measurement setup is complicated and intrusive, thus not practical for longer term or home use. Therefore, there is a need for alternative and easier ways to detect and characterize sleep disorders.